The invention relates to a proximity fuse for projectiles, missiles or the like comprising means for transmission of an electromagnetic wave and reception of an electromagnetic wave, which is re-transmitted after reflection from an object. The re-transmitted wave is combined with the transmitted one for generation of a doppler signal, which is fed to an ignition circuit via a signal processing circuit, in which the dynamic variation of the doppler signal is determined by comparing the prevailing value of the doppler signal with the value of the doppler signal a given time interval previously.
Such a proximity fuse is known from U.S. Pat. No. 4,192,235, where the signal processing circuit consists of an automatic gain control circuit for a receiver amplifier comprising a delay filter connected in the feed-back loop. Because the gain control takes place with time delay from the output of the amplifier, this circuit will produce an output signal which is an approximative value for the quotient of the prevailing value of the amplitude of the doppler signal divided by its amplitude a given time interval previously. This quotient varies in a characteristic manner with the distance to the reflecting object and can consequently be used for initiating the ignition circuit at a desired distance from the object. The dynamic variation of this quotient is, however, dependent upon the approaching speed of the projectile toward the object and in order to be able to initiate the ignition circuit at a desired distance it is consequently necessary to know this approaching speed. Furthermore, the automatic gain control circuit in the known device gives only an approximative value of the quotient between the amplitude of the prevailing doppler signal and its amplitude said given time interval previously. The accuracy of the appoximation depends i.a. on the time constant in the control circuit in relation to the dynamic behaviour of the doppler signal, which in turn is dependent upon the approaching speed.
The known circuit therefore operates in principle only for one single approaching speed in its desired function to give a constant distance of burst. It is assumed in the patent specification that this approaching speed is constant and known.